Strain gages are used for precision measurement of force, weight, pressure, torque, displacement and other mechanical quantities that can be converted to strain in a mechanical member. A strain gage transducer system includes one or more sensing units, such as load cells, with connection means, interconnecting cables, junction boxes, and electronic equipment for supplying power to the sensing units and for amplifying and/or converting the signals from sensing units to useful form, such as a visual display or input signals for printers or computers.
A strain gage sensing unit includes a mechanical member, such as a beam or membrane, on which the quantity to be measured acts, and typically two or four strain gages bonded to the member in such a way that they sense strain variations in the member. The strain gages are connected together to form a Wheatstone bridge, with one diagonal of the bridge connected to a voltage source and the other bridge diagonal providing an output signal. A strain gage sensing unit also typically includes temperature compensating resistors, zero adjusting means, calibrating resistors, and sometimes linearizing elements, all connected as part of the bridge circuit. The output signal from a strain gage bridge at 100% signal is typically about 20 mV for a sensor with 10 V supply voltage, and the resistance of each bridge arm is typically 350 ohm or 1,000 ohm.
A major problem with strain gage transducer systems is errors caused by moisture and other contaminants appearing as shunt resistance over a bridge arm. Such shunt resistances can cause large zero shifts that are very unstable and unpredictable, so they limit the accuracy of the system.
To reduce the moisture problem, strain gages are potted, or the sensing units with strain gages inside are sealed hermetically. Moisture can, however, still affect the insulation resistances in external cable connections to the sensing units, as well as the insulation resistance in junction boxes, and the insulation resistance between individual conductors in cables. To reduce the effect of moisture outside the sensing units, sensing units have been built with buffer amplifiers inside. It has even been proposed to include a microprocessor inside each sensing unit, so only low-impedance or digital signals are transmitted in the system. Such solutions obviously increase the complexity and cost of the sensing units and the total system cost.